Histological, ultrastructural and chromatographical discrimination of phospholipids in meningiomas

Calpain-dependent proteolysis of merlin occurs by oxidative stress in meningiomas: a novel hypothesis of tumorigenesis

BACKGROUND

The purpose of this study is to indicate that oxidative stress may contribute to occurrence of meningiomas. Recently, it was reported that aside from the neurofibromatosis type 2 (NF2) gene mutations, the calpain-dependent proteolysis of the NF2 gene product, merlin might be closely related to the development of certain NF2-related tumors. Although meningiomas are well known to occur more frequently in aged persons, it still remains unknown why calpain activation occurs predominantly in them. Because the production of free radicals with aging might be one of the causes of calpain activation especially in leptomeningeal cells being devoid of blood supply, the authors examined the relations between mu-calpain activation and merlin proteolysis induced by the oxidative stress.

METHODS

The authors examined 12 patient-derived sporadic meningiomas and their primary cultured cells. Malignant glioma cell line (U-251MG), which had no relation to NF2, was used as a control. They were exposed to hydrogen peroxide (H2O2) for 1 hour. After oxidative stress, they were examined by Western blot and immunofluorescence microscopic analyses.

RESULTS

Despite the consistent expressions of activated mu-calpain in 11 of 12 meningioma tissues, this calpain activation completely disappeared after culture; instead the full-length merlin appeared again in 8 of 11 cases. The treatment of cultured cells with hydrogen peroxide induced both mu-calpain-dependent cleavage of merlin and reduction of an intrinsic calpain inhibitor calpastatin. Such proteolysis was significantly blocked by a specific calpain inhibitor, Z-LLal. The full-length merlin was immunocytochemically colocalized with activated mu-calpain at the plasma membrane, and, after mu-calpain activation, the fragment of merlin translocated to the perinuclear cytoplasm or into the nucleus.

CONCLUSIONS

These findings suggest that oxidative stress-induced activation of mu-calpain causes proteolysis of merlin conceivably to impair cell adhesion and/or contact inhibition of meningioma cells.

On Arachnoid Villi and Meningiomas: Functional Implication of Ultrastructure, Cell Adhesion Mechanisms, and Extracellular Matrix Composition

Arachnoid villi or granulations are small projections of the arachnoid barrier layer into the venous sinus and its major tributaries. They are closely related to the absorption of cerebrospinal fluid, and are widely accepted to be the origin of human meningiomas. Arachnoid villi and meningiomas show a number of similarities in ultrastructure, cell adhesion mechanisms, and extracellular matrix composition. Ultrastructurally, both arachnoid and meningioma cells are characterized by interdigitations connected with junctional complexes, and extracellular cisterns related to the fluid transport. Extracellular cisterns and the intercellular space reveal abundant membrane-derived multilamellar phospholipids when a conventional ultrastructural fixative supplemented with tannic acid is used. Both arachnoid and meningioma cells are connected by Ca2+-dependent adhesion molecules: epithelial-cadherins which are concentrated at the adherens junctions. Membrane-cytoskeleton interactions by means of merlin and a-catenin molecules are thought to be crucial in signal transduction resulting in contact inhibition of cell growth in normal arachnoid cells. Impairment of these molecules might be related to meningioma-genesis. Glutathione-independent prostaglandin D2 synthase [EC 5.3.99.2] responsible for the biosynthesis of prostaglandin D2 in the central nervous system is also consistently expressed in human arachnoid villi and meningiomas. The multilamellar phospholipids are conceivably related to this arachidonate metabolism.

Prostaglandin D synthase (beta-trace) in human arachnoid and meningioma cells: roles as a cell marker or in cerebrospinal fluid absorption, tumorigenesis, and calcification process

Glutathione-independent prostaglandin D synthase (PGDS) is an enzyme responsible for biosynthesis of prostaglandin D2 in the CNS and is identical to a major cerebrospinal fluid protein, beta-trace. Although PGDS has been identified recently in rat leptomeninges, little information is available about human meninges or meningiomas. Here, we report PGDS to be expressed consistently in 10 human arachnoid and arachnoid villi and in 21 meningiomas by immunohistochemistry, Western blot, and reverse transcription (RT)-PCR analyses. In arachnoid, PGDS immunoreactivity was seen in arachnoid barrier cells but was negligible in arachnoid trabecula and pia mater. In contrast, in arachnoid villi, PGDS was seen in core arachnoid cells rather than in the cap cell cluster or arachnoid cell layer. Meningioma cells also showed intense immunoreactivity in the perinuclear region, and it was often concentrated within meningocytic whorls and around calcifying psammoma bodies. Immunoelectron microscopic data, when compared with the ultrastructure, showed that PGDS was localized at rough endoplasmatic reticulum of arachnoid and meningioma cells. Western blot showed a 29 kDa immunoreactive band indicating PGDS, but the extent of expression was variable from case to case, which was compatible with immunohistochemical data. RT-PCR revealed PGDS gene expression in all meningiomas studied, regardless of histological subtypes, and also in human arachnoid villi. Because human arachnoid and meningioma cells exclusively express PGDS, it can be considered their specific cell marker. These results show functional differences in various types of meningeal cells attributable to differences in PGDS expression.